Electrolytic recovery of zinc



Patented Feb. 7, 1950 ELECTROLYTIC RECOVERY OF ZINC Dinsmore Laurence Griffith and Eldon Irwin Isherwood, Flin Flon, Manitoba, Canada, assignors to Hudson Bay Mining and Smeltlng Co., Limited, Winnipeg, Manitoba, Canada, a corporation of Canada No Drawing. Application October 15, 1945, Serial No. 622,480. In Canada September 4, 1945 1 Claim.

This invention relates to the deposition of metals by electrolysis when use is made of anodes containing lead.

In the electrolytic production of metals the metal to be produced is contained in solution in the electrolyte and is deposited on the cathode. For certain metals such as zinc, cadmium and copper it is either necessary or desirable to make use of anodes consisting of lead or containing a substantial proportion of that metal. During operation of the electrolytic cell, lead tends to be transferred from anode to the cathode. This transfer is to a great extent controlled by the condition of the surface of the anode: during electrolysis this becomes coated with a layer of a lead oxide, probably the peroxide. If this layer is thin and coherent the transfer of lead is slight, but if it is thick and coarse-grained the transfer becomes great. The deposit of lead on the cathode is always undesirable and is particularly so in the case of the electrolytic recovery of zinc from zinc sulphate electrolytes since zinc for certain purposes, notably die casting, is required to be substantially free from lead, specifications commonly laid down callin for a maximum of not more than from .003% of lead in the zinc in some cases and in others for a maximum of not more than .007%.

We have discovered that the presence in the electrolyte during the process of electrolysis of small quantities of silver tends to improve the surface condition of lead anodes and to reduce the quantity of lead passing therefrom to the cathodes. The silver may be introduced into the electrolyte in any form desired, but we have found that its introduction in the form of sulphate gives satisfactory results.

The quantity of silver to be added depends upon the metal to be recovered but in the case of zinc as little as 5 milligrams of silver to each litre of electrolyte tends to improve the surface condition of the lead anode and to reduce the quantity of lead passing therefrom to the cathode. The improvement in these respects increases with increased quantities of silver but the improvement secured is not great in proportion to the amount of silver added if this exceeds 20 milligrams per litre. In the case of cadmium larger amounts up to 30 milligrams to each litre of electrolyte and in the case of copper 20 milligrams to each litre have been found to give satisfactory results. It is desirable that the quantities of silver should not be greater than is necessary, not only on account of the expense involved, but also because some silver tends to 2 be deposited on the cathodes with the metal to be recovered and the amount so deposited increases as the amount of silver is increased. The silver, however, is not, in the case of zinc and most other metals, a deleterious impurity.

For example, we have found that using an electrolyte of the following composition:

Zinc grams per litre 124.0 Copper milligrams per litre 1.5 Cadmium do 0.3 Arsenic d0 0.03 Antimony do 0.02 Chlorine do 4.5 Iron do- 35. Cobalt d 0.4

90 parts per million (P. P. M.) of lead was included with the zinc on the cathodes, whereas this quantity was reduced to '71 P. P. M. by the addition to the electrolyte of 5 milligrams of silver per litre, to 28 P. P. M. with the addition of 10 milligrams per litre and to 11 P. P. M. with the addition of 20 milligrams per litre. So with an electrolyte prepared from cadmium sulphate of a chemically pure grade in a concentration of grams per litre, a reduction of in the percentage of lead deposited with the cadmium on the cathodes was found to occur when there were 30 milligrams of silver sulphate added to each litre of electrolyte.

In the recovery of zinc or copper the introduction of the silver into the electrolyte in quantitles of the order indicated has no substantial effect on the average current efficiency. The

' variation between the case in which no silver was added and that in which 20 milligrams per litre were added was only slightly over 1 In the case of cadmium, however, the loss of current efiiciency was more substantial but not so great as to offset the increased purity of the recovered metal.

What we claim is:

In the electrolytic recovery of zinc from a zinc sulphate electrolyte when using an anode of which a substantial proportion is lead, the

method of obtaining zinc at the cathode containing less than about 007% of lead comprising introducing into the electrolyte from about 5 to about 20 milligrams of silver per litre of electrolyte and carrying out the electrolysis with such silver present in the electrolyte.

DINSMORE LAURENCE GRIFFITH.

ELDON IRWIN ISHERWOOD.

(References on following page) REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,587,438 Tainton June 1, 1926 1,739,772 Tainton Dec. 17, 1929 1,759,493 Tainton May 20, 1930 4 Number Name Date 1,782,092 Gray Nov. 18, 1930 1,934,305 Hannay et a1 Nov. 7, 1933 2,042,591 Doran et a1 June 2, 1936 2,419,722 Lowe et a1 Apr. 29, 1947 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 75 (1939), pages 187, 431, 435. 

